Manufacturers of consumer electronic devices such as mobile phones and laptops are turning to glass as the material of choice for device covers. These device covers are regularly smudged with fingerprints and other materials due to normal use. Therefore, it is desirable to make the covers easy to clean. One approach to making covers that are easy to clean is to coat the surface of the glass used in making the covers with a fluorosilane compound. The fluorosilane compound could reduce the surface energy of the glass, which would result in an increase in the contact angle of many materials, such as water and oil, on the glass. This increase in contact angle means that the treated glass surface will be easy to clean.
Examples of a fluorosilane compound suitable for use as an anti-smudge coating are Dow Corning® 2604 and 2634 coatings (alkoxysilyl perfluoropolyether adducts). Normally, HFE7200 solvent (ethyl nonafluorobutylether), which has a boiling point of about 76° C., is mixed with the coating material to form a coating solution. Dow Corning Corporation recommends applying these coatings on bare glass with a standard dip process or a chemical vapor deposition process. With the proper application of the coating on the glass surface, large static contact angle (>100° for deionized water, >70° C. for oleic oil) and low sliding angles can be achieved. Also, the performance of the coating will not degrade even after 10,000 wipes on the surface (Block, Steven et al., New Anti-Fingerprint Coatings Whitepaper, Dow Corning Corporation, 2008). The easy-to-clean surface coating does not need to be excessively thick; only a monolayer of the material (about 2 nm) is needed on the glass surface to provide the desired performance. On the other hand, a thicker multilayer of the coating material does not increase the cleaning performance and may leave an undesirable visible stain.
The standard dip coating process is widely used in coating a surface. The process can be easily implemented and does not require complex process equipment. Also, the process can be designed to coat a large number of samples at one time, which has a positive impact on throughput. However, the dip coating process has several drawbacks. During the dip coating process, a large amount of coating solution is exposed to the ambient environment, resulting in significant loss of the coating material due to evaporation. This lowers effective use of the coating material and increases the cost of the coated glass. As each dip coating is performed, the solution in the dip coating bath is susceptible to contamination, which may lead to contamination of the coated glass. The dip coating process results in a thicker-than-required (possibly non-uniform) coating layer, with excess coating material on the glass. This leads to visible stains on the glass surface that must be removed. Typically, these coating stains are removed by rinsing the stained glass in a bath containing the original (or another fluorocarbon) solvent. This rinse bath is also typically exposed to the environment, leading to evaporation of solvent, increase in material cost, and release of volatile fluorocarbon to the atmosphere. In addition, since the dip coating process involves submerging the glass in a bath, the dip coating process typically coats all sides of the glass, whereas only one side requires the easy-to-clean coating. Thus, the coating on the other side of the glass needs to be removed. This typically requires use of plasma or UV ozone. This extra coating leads to more process steps (i.e., to remove the material from undesired sides/areas of the glass) and a higher product cost (i.e., more coating material is used than is needed).